1. Field of the Invention
The present invention relates generally to the field of current sensors and methods for motor control. More specifically, the present invention relates to an improved current sensor and current sensor components for motor control.
2. Background of the Prior Art
In the field of motor control circuitry, there is always a system performance related issue of how to get motor phase current information accurately and instantly to the motor control unit.
The motor control unit is generally a low power circuitry configuration comprising a digital signal processor and/or microcontroller. In contrast, the motor phase currents are generated by a high voltage switching mode power supply. Such voltages are too high to be connected directly to the motor control unit, otherwise such voltages would cause damage to the low power circuitry of the motor control unit. As a result, various current sensors have been developed in the prior art to isolate high voltage switching mode power supply from the motor control unit while transferring the high side motor phase current information to the motor control unit at low voltage DC power supply.
One such prior art sensor control circuit is illustrated in FIG. 1, wherein the current sensor 11 is shown for one of the normally three or more phases of the motor. A high voltage power supply is connected to switching components 1 and 2 with its terminals Vbus+ and Vbus−, respectively, as shown in FIG. 1. Both switching components 1 and 2 can be turned on or off depending on the motor control algorithms. However, switching components 1 and 2 may never be on at the same time.
Turning now to the operation of this prior art current sensor, a signal conditioner 7, powered by a floating positive supply 5, is connected directly to a shunt resistor 4 whereas the current coming from switch components 1 or 2 flows through shunt resistor 4 to the motor 3. Measurement unit 8 measures the output signal of signal conditioner 7. Because the measurement unit 8 is in high side too, the low side motor control unit 10 can not receive output from measurement unit 8 directly.
To solve above problem, an isolator 9 is introduced. We also call it signal transfer unit 9 because it does both isolation and signal transferring work in one. After signal transfer unit 9, the output voltage or data from isolator 9 can be accepted by motor control unit 10.
An existing example circuit like that shown in FIG. 1 is the IR2277S/IR2177S IC circuit from International Rectifier located at 233 Kansas Street, El Segundo, Calif. 90245. The timing diagram 12 of transferring signal in this way is shown in FIG. 2.
The parameters of FIG. 2 are as follows:
                Vmax: Maximum input voltage before input saturation (around +250 mV)        Vmin: Minimum input voltage before input saturation in reverse direction (around −250 mV)        Vin: Actual input voltage coming from shunt resistor        Sync: Low side input synchronization signal from control unit        PO: PWM Output pulse signal        Vrh: OUT Reference High Voltage        Vrl: OUT Reference Low Voltage        OUT: Analog Output Voltage        
As shown in FIGS. 1 and 2, the current measurement is performed on the high side in this circuit. The output is a low side pulse-width modulated (PWM) signal that represents the shunt current level. One disadvantage of this prior art circuit is that the working frequency is limited to maximum of 20 kHz. Another disadvantage is that it needs an external synchronous signal from control unit 10 that must be at most equal to or lower than switching frequency (20 KHz). When the shunt current changes to a new value, it also needs two synchronous cycles to stabilize the new output.
The disadvantages of this prior art system is the complexity and low speed of the current sensor 11. Whereas motor control units already exist that are capable of measuring current from a low power input signal, it would be advantageous to provide an improved current sensor that eliminates the signal measurement component 8 as required by this prior art circuit 11.
Another existing example of a prior art motor current sensor utilizes isolated digital communication as shown in FIG. 3. An example of this prior art device is the HCPL-7860 from Hewlett Packard at 3000 Hanover Street, Palo Alto, Calif. 94304-1185 USA . The circuit 15 carries out three tasks, signal conditioning, signal measurement and signal transfer in one isolated modulator IC 13 which converts shunt voltage input 16 into digital data and sends out data serially synchronized by its clock output (10 MHz typical) pin. The data signal from the isolated modulator 13 can not be accepted by a normal digital control unit 17, so a second digital interface IC 14 is needed, dedicated to digital communication between isolated modulator 13 and digital control unit 17.
Each shunt channel 16 needs one digital interface 14. The overall cost and circuit complexity are thus increased due to more digital interfaces 14 needed for multi channel shunts in multiphase motor control.
Another disadvantage of this prior art circuitry is its resolution/speed trade-off. An 18 us time delay occurs when 12 bit resolution is transferred, and a 94 us signal time delay occurs for 14 bit resolution transformation. These time delays compromise too much performance in, e.g., 20 khz switching frequency motor control.
At least in view of the above, it would be desirable to reduce the complexity of the circuitry between shunt resistor 16 and digital control unit 17 while maintaining accuracy at high switching frequency. It would also be advantageous to increase the circuit reliability with less signal delay time than this prior art circuit.
It is an objective of the present invention to overcome these and additional disadvantages of the prior art current sensors for motor control and to provide additional improvements which shall become apparent in the summary, drawings and detailed description of the present invention.